The invention relates to a process for recovering heat from the combustion of anode waste gas produced in fuel cells. More specifically, the invention relates to a process for the multi-stage combustion of anode waste gas in fuel processors to supply heat to a fuel processor for converting a combustible fuel such as methane into a hydrogen-rich fuel for use in a fuel cell.
The operation of fuel cell power generators for stationary and mobile use comprises a fuel processor and a fuel cell which are integrated to efficiently convert a hydrocarbon or alcohol feedstock into a hydrogen-rich fuel. Typically, the conversion of the feedstock occurs in a fuel processor wherein either a reforming reaction or a partial oxidation reaction takes place in the presence of steam to produce hydrogen and carbon oxides. The reforming process, often called steam reforming, is an endothermic chemical reaction which requires a significant amount of heat to drive the reforming reaction toward the production of hydrogen. The partial oxidation reaction is an exothermic reaction which requires the removal of heat for the equilibrium reaction to favor the production of the hydrogen-rich fuel for the fuel cell. Often a fuel processor will employ some combination of the steam reforming reaction and the partial oxidation reaction to provide heat to the steam reforming reaction from the heat released in the partial oxidation reaction. The partial oxidation reaction is typically carried out in the presence of an oxygen-containing gas and an oxidation catalyst selective for the production of hydrogen and carbon oxides.
Fuel cells are chemical power sources in which electrical power is generated in a chemical reaction. The most common fuel cell is based on the chemical reaction between a reducing agent such as hydrogen and an oxidizing agent such as oxygen. The consumption of these agents is proportional to the power load. Because hydrogen is difficult to store and distribute and because hydrogen has a low volumetric energy density compared to fuels such as gasoline, hydrogen for use in fuel cells will have to be produced at a point near the fuel cell, rather than be produced in a centralized refining facility and distributed like gasoline. Polymers with high protonic conductivities are useful as proton exchange membranes in fuel cells. Among the earliest proton exchange membranes were sulfonated, crosslinked polystyrenes. More recently sulfonated fluorocarbon polymers have been considered. Such proton exchange membranes are described in an article entitled, xe2x80x9cNew Hydrocarbon Proton Exchange Membranes Based on Sulfonated Styrene-Ethylene/ Butylene-Styrene Triblock Copolymersxe2x80x9d, by G. E. Wnek, J. N. Rider, J. M. Serpico, A. Einset, S. G. Ehrenberg, and L. Raboin presented in the Electrochemical Society Proceedings (1995), Volume 95-23, pages 247 to 251.
The fuel cell operation comprises passing the hydrogen-rich feed stream from the fuel processor to the anode side of the fuel cell and simultaneously contacting the cathode side of the fuel cell with an oxygen-containing stream, typically air, for the production of electricity within the fuel cell. An anode waste gas comprising hydrogen and a cathode waste gas comprising oxygen are also produced as by-products by the fuel cell during the electricity generation process. The anode gas has fuel value and is typically returned to the fuel cell processor for further hydrogen enrichment or for combustion to generate heat for the reforming process. The efficient operation of a fuel cell system requires the balance of the overall energy demands of the fuel processor. The cathode waste gas is generally oxygen-lean and is often employed to moderate combustion.
In a fuel cell system, the operation of the fuel cell is dependent upon the external demand for electric power. When the demand for power is high, the fuel processor must supply hydrogen-rich fuel to the fuel cell. The fuel cell, in turn, produces the anode waste gas which in the most basic scheme is burned to provide heat for the fuel processor. As the electric power demand on the fuel cell varies, variations in the hydrogen content, and hence the heating value of the anode waste gas occur. The response of the fuel cell is relatively rapid to a reduction in the supply of the hydrogen-rich fuel. However, the fuel processor does not respond as quickly to variations in electric power demand. A variation in the heating value of the anode waste gas can reduce the efficiency of the fuel processor and significantly increase the overall energy cost of producing the electricity from the fuel cell. In addition, the variation in electric power demand results in large temperature swings within the fuel processor which can create thermal stress on heat exchange equipment and lead to premature failure or fire.
In an attempt to solve the two-sided problem of varying anode gas quality and the need to recover the energy of the anode waste gas, others have employed special combustion zones. These combustion zones operate in intimate thermal contact with the fuel processor to provide heat to the reforming reaction zone. One such approach uses a single-stage combustion zone. In a single-stage combustion zone, the temperature of the combustion gases leaving the combustion zone is controlled by the rate of air, or the amount of excess air, which is introduced to the combustion zone as the fuel is burned. The combustion gases withdrawn from the combustion zone are generally passed to the fuel processor to provide heat to the reforming zone. The temperature of the combustion gases, or combustion zone effluent, establishes the level of conversion in the reforming zone, and also establishes the type of metallurgy in the heat transfer zone between the hot combustion gases and the reforming zone. In many fuel cell/processor systems using simple, single-stage combustion zones, the combustion zone comprises a single burner which is surrounded by an annular reforming zone to obtain the maximum amount of energy from the combustion process. Examples of such arrangements may be found in U.S. Pat. No. 5,110,559, U.S. Pat. No. 4,925,456, U.S. Pat. No. 5,181,937, and U.S. Pat. No. 4,861,348. Exotic metallurgy is employed in the combustion zone where heat transfer takes place at temperatures above 800xc2x0 C.
U.S. Pat. No. 5,609,834 attempts to minimize the amount of excess air passed to the combustion zone by balancing and directing the heat provided to the reforming zone by means of an internal combustion zone which can adjust the magnitude and location of the combustion temperature within the combustion zone. The result is a more efficient operation wherein more useful heat is transferred to the reforming zone and the amount of excess air in the combustion zone is reduced. However, in practice, it is difficult to balance the heat input with the heat exchange because the location and magnitude of the peak temperature within the reforming zone varies with the plant capacity or electric power demand. Furthermore, it is difficult to direct the anode waste gas to this exact point in the combustion zone without imposing a large pressure drop in the anode waste distribution. In this scheme, the excess air stream is employed to control the flue gas temperature leaving the reformer rather than the temperature of the flue gas entering the reforming zone. A complex, advanced control scheme, along with a plurality of exotic temperature sensing elements in the combustion zone are required to identify the maximum temperature in the reforming zone and to avoid exceeding the maximum temperature to prevent damage to the reforming zone equipment. Since relatively cool air is employed to control the flue gas temperature, large thermal stresses are introduced when the air flow rate is suddenly increased to compensate for an increase in the heating value of the spent anode gas. U.S. Pat. No. 5,776,421 recognized the difficulty in controlling the resulting thermal stress and sought to reduce thermal stress by arrangement of the reforming reactor and burner internals.
Processes are sought for combusting the anode waste gas which simplify the overall process of burning the anode waste gas and improve the operation of the reforming zone. It is an objective of the present invention to improve the efficiency of the fuel processor for the conversion of hydrocarbons and oxygenate compounds such as alcohols, ketones, ethers, and the like.
It is an objective of the present invention to provide a process for the combustion of anode waste gas which moderates temperature swings in the fuel processor.
It is an objective of the present invention to minimize reforming temperatures while recovering heat from the combustion of anode waste gas.
The operation of fuel processors in conjunction with fuel cells, wherein the anode waste gas is burned to provide a portion of the heat to produce hydrogen, is subject to wide swings in temperature as a result of variations in anode waste gas heat content which varies in response to electric power demand. The present invention provides a simple solution to the problem of balancing energy demand with fluctuating anode waste gas heating value without requiring exotic materials of construction in the reforming zone to cope with high reforming temperatures and rapid temperature swings. The invention further provides a simple, direct control system for adjusting the reforming temperature which is based on the rate of air supplied to the combustion zone. The present invention does not require advanced control methods or exotic flame temperature measurement methods. The resulting process represents an economical and reliable integration scheme for operating a fuel processor in combination with a fuel cell for the generation of electricity.
In one embodiment, the present invention provides a process for reforming a feedstock in the presence of steam in a reforming zone in connection with a fuel cell. The process comprises passing the feedstock and steam to the reforming zone to produce a reformate stream comprising hydrogen. The reforming zone is at least partially heated by indirect heat exchange with a first combustion effluent stream to provide a cooled first combustion effluent stream. The cooled first combustion effluent stream is reheated by recombusting the cooled first combustion effluent stream with at least a portion of an anode waste gas stream withdrawn from the fuel cell to provide a second combustion effluent stream. The reforming zone is further heated by indirect heat exchange with the second combustion effluent stream.
In a further embodiment, the present invention provides a process for reforming a feedstock in the presence of steam in a reforming zone in connection with a fuel cell. The process comprises passing the feedstock comprising a hydrocarbon or an oxygenate and steam at effective reforming conditions to a reforming zone containing a steam reforming catalyst to produce a reformate stream comprising hydrogen. The reforming zone is heated by indirect heat exchange with a first combustion effluent stream to provide a cooled first combustion effluent stream. The cooled first combustion effluent stream is reheated by recombusting the cooled first combustion effluent stream with an auxiliary fuel stream to provide a second combustion effluent stream at a second combustion temperature. The reforming zone is further heated by indirect heat exchange with the second combustion effluent stream. Essentially all of an anode waste gas stream withdrawn from the fuel cell is combusted with an excess air stream to provide the first combustion effluent stream at a first combustion effluent temperature essentially equal to the second combustion temperature.
In a still further embodiment, the present invention provides a process for controlling a fuel processor used in conjunction with a fuel cell wherein said fuel processor comprising at least two combustion zones and a reforming zone, wherein a portion of an anode waste gas stream withdrawn from the fuel cell is combusted with an excess air stream at an excess air rate in a first combustion zone to provide a first combustion effluent stream at a first combustion temperature and wherein the first combustion effluent stream is cooled by indirect contact with the reforming zone to produce a cooled first combustion effluent and wherein said cooled first combustion effluent is recombusted in a second combustion zone with at least a portion of the anode waste gas stream to provide a second combustion effluent stream at a second combustion temperature, said process comprises measuring the first combustion temperature and adjusting the excess air rate to maintain a uniform first combustion temperature.